Sensitivity of the Brightness Temperature to Soil Moisture as a Function of Depth, Frequency, Polarization and Incidence Angle

ESA SMOS and NASA SMAP missions are measuring surface soil moisture (SM) globally from space using L-band microwave radiometers. Many studies have been conducted to validate the quality of these SM retrievals using data from different SM networks. However, neither the sampling is dense enough, nor the terrain is homogeneous enough within the pixel footprint. There is also experimental evidence that, under the same irrigation conditions, the vertical SM profiles are different because of the soil type, composition, and porosity. Therefore, the L-band brightness temperature measured by the radiometers is sensitive to the SM at different depths, which limits the accuracy of the validation. This study attempts to address this question by analyzing theoretically the contributions to the brightness temperatures of a SM layer at different depths. The study includes the effects of the frequency, polarization, incidence angle, and background SM value.